2.1 Study design and setting
This retrospective study was conducted using data provided by JMDC Inc.
(Tokyo, Japan), a commercial healthcare data vendor [11,12]. The
JMDC database includes data from >100 society-managed
health insurance plans covering employees and their dependents inclusive
of children. The cumulative number of enrollees since 2005 is
approximately 5 million. The study period was between December 1, 2018
to March 31, 2019. In Japan, during the 2018/2019 influenza season, the
dominant circulating virus was A (H3N2) strain (56.0%), followed by A
(H1N1) strain (36.5%) and all types of B strains (7.5%) [13]; this
represented lower influenza B activity than had been observed in the
preceding seasons.
This study protocol was approved by the ethics committee at Kyoto
University, including waiver of informed consent from each patient.